In wireless communications, the physical channel between the transmitter and the receiver is formed by a radio link. In most cases, many different propagation paths exist between the transmitter and the receiver. This is due to reflections in the environment, for example, against buildings, and gives rise to a multipath channel with several resolvable components for each transmitter.
The performance of a Code Division Multiple Access (CDMA) receiver is improved if the signal energy carried by many multipath components is utilized. This is traditionally achieved by using a RAKE receiver. A detailed description of RAKE receivers may be found, for example, in the book “WCDMA for UMTS” by Holma, & Toskala, A. (Wiley 2000).
A RAKE receiver includes usage of various ‘fingers’, or despreaders, each finger having an assigned path delay for receiving a particular image of a multipath radio signal and a correlator for despreading the received image. In combination, the fingers despread multiple signal images of a received multipath radio signal, thus mitigating the effect of the multipath channel fading phenomenon. In other words, in a RAKE receiver, each multipath component is assigned a RAKE finger, or despreader, whose reference copy of the spreading code is delayed equally to the path delay of the corresponding multipath component. The outputs of the RAKE fingers are then coherently combined in order to produce a symbol estimate. As shown in FIGS. 1A-B, this is performed by the RAKE receiver by utilizing its knowledge of the channel response (dashed line) for all paths h0, h1, h2 and corresponding multipath delays d0, d1, d2. The channel estimation of each path in the multipath radio signal being directly determined based on channel response.
The combining of the outputs in the RAKE receiver improves the signal-to-noise ratio (SNR) since it allows the desired signal components to be summed coherently, while the interference and noise components are summed non-coherently. When the noise components are uncorrelated at each RAKE finger, they partially cancel each other out, while the signal components are rotated so as to sum constructively. The combining in the RAKE receiver is normally performed by weighting the different signal components according to their conjugated channel response at their respective delay. RAKE receivers normally work well when the spreading factors (SF) are large, for example, a ratio of 256, which means that there is only a small amount of correlation between the different RAKE fingers.
In WCDMA, a requirement added in the 3GPP standard (in Release 6) was the Enhanced Uplink (EUL). With the introduction of EUL, the spreading factors may be as low as a ratio of 2, which means that there may be a significant amount of correlation between the different RAKE fingers. In these cases, the achievable SNR by the RAKE receiver is limited mainly by the interference from other users, such as, Multi User Interference (MUI), and the self-interference of the user, such as, Inter Symbol Interference (ISI) and Inter Channel Interference (ICI).
For dealing with this situation, Generalised RAKE (GRAKE) was designed. GRAKE receivers estimate the correlations between the RAKE fingers and use these correlations to modify the weights of the different signal components. Besides using the RAKE fingers that collect signal energy of the multipath radio signal received at the antenna, a number of “interference suppression” fingers are added to collect information about the interference on the RAKE fingers and used to cancel this interference. The locations of the “interference suppression” fingers may, for example, be based on the self-interference of the specific user. Furthermore, in addition to weights based on the estimated channel response as in a RAKE receiver, GRAKE receivers also uses the correlation between the impairment (interference plus noise) on different fingers in order to suppress interference.
It should also be noted that while WCDMA was originally designed for many low data rate (LDR) users, that is, users using voice and data services having low data transmission rates, such as, for example, voice calls and text messaging, the number of high data rate (HDR) users, that is, users using data services having high data transmission rates, such as, for example, video calls and video streaming, is constantly increasing. This new usage has also added new requirements on the uplink.